An inorganic hydrated oxide, precipitated from an aqueous solution of a corresponding metal cation, washed and then dried (in air or under a vacuum) yields a porous oxide product. The porous oxide product, produced from the gel (the precipitated inorganic hydrated oxide in the solvent) by removing the solvent (water) under conditions of temperature and pressure less than the supercritical conditions (at or above which the solvent would exist as a supercritical fluid), has been given the name "xerogel" by A. Fruendlich (Colloid and Capillary Chemistry, Dutin, N.Y., 1923). However, the textural characteristics (pore volume and surface area) of the xerogel are inferior to the textural characteristics of the oxide in the gel before the elimination of the solvent. It is theorized that the vaporization of the water creates a vapor-liquid interface inside the pores of the oxide particles and the resulting surface tension is responsible for a partial collapse of the pores. Nonetheless, xerogels are very useful products and have been the subject of a number of patents. For example, U.S. Pat. No. 4,226,743 (Seese et al.) describes a process for the production of a silica-alumina hydrogel catalyst, U.S. Pat. No. 3,383,172 (Biegler et al.) describes a process for the production of silica from an aqueous suspension, and U.S. Pat. No. 3,704,147 (Hardy et al.) describes a process for the production of inorganic oxides such as zirconia or alumina from an aqueous suspension.
It is known to produce the hydrated inorganic oxides by hydrolysis of metal alkoxides. The hydrated oxides which precipitate are removed from the aqueous suspension by, for example, filtering or centrifugation, and are subsequently washed and dried in ovens to yield the oxide. Such processes have been described in "Preparation, Characterization, and Comparison of Properties of Alumina Catalysts," J. Catal., 89, 560-563 (1984), Matijevic, Egon, "Monodispersed Metal (Hydrous) Oxides--A Fascinating Field of Colloid Science," Acc. Chem. Res., 14, pp. 22-29 (1981), and Barringer, E. et al., "Processing Monosized Powders," Ultrastructure Processing of Ceramics, Glasses, and Composites, John Wiley & Sons, New York, N.Y., pp. 315-333.
The step of drying solutions or suspensions to produce particulate materials has been accomplished by a variety of processes. Such processes include oven drying, flame pyrolyzing, and spray drying. Of these processes, spray drying has received considerable attention. Spray drying as defined herein includes flash drying and is a technique in which a solution or suspension is supplied to a heated chamber as a plurality of uniform, fine droplets to remove the solvent. The product produced therefrom is ordinarily spherical and finely divided. The process of spray drying aquagels is disclosed in U.S. Pat. Nos. 4,226,743 (Seese et al.); 4,389,385 (Ramsey); 4,407,967 (Luks); 4,297,163 (Thomas); and 2,856,268 (Young).
The process of spray drying aquagels formed from the hydrolysis of metal alkoxides to produce xerogels is also known. U.S. Pat. No. 4,407,734 (Denton et al.), U.K. Pat. No. 1,567,003 (Ramsey) and a sales brochure from Conoco entitled "Catapal.RTM.SB Alumina" describes such processes. The pore volume and surface area of alumina xerogels prepared by these techniques are generally less than about 1.7 cm.sup.3 /g and 260 m.sup.2 /g, respectively.
Modifications to the basic processes for producing xerogels have been employed to prepare inorganic products having higher pore volume and surface area. One such method involves the use of additives in the gel. For example, control and enlargement of the pore volume of alumina by the addition of water-soluble polymers is described in J. Catal., 1, 547 (1962). Incorporation of carbon black is disclosed in German Pat. No. 1,907,095, the addition of alcohol is disclosed in EPA No. 0050902, and the addition of polyethylene glycol is disclosed in U.S. Pat. No. 4,145,316. However, the pore volume and surface area of, e.g., alumina prepared using any of these aqueous techniques is generally less than about 2 cm.sup.3 /g and 300 m.sup.2 /g, respectively.
An alternate approach employed to create high pore volume and surface area products is a process of evacuating the solvent from a gel under supercritical conditions This process was developed, in part, and reported by Kistler (J. Phys. Chem., 36, 1932, p. 52). Products produced by this process are classified as aerogels. Aluminum oxides produced by this process readily exhibit a pore volume above 2 cm.sup.3 /g and a surface area greater than about 400 m.sup.2 /g. A detailed discussion of the method of preparing inorganic oxide aerogels is disclosed in S. J. Teichner et al., "Inorganic Oxide Aerogels," Advances in Colloid and Interface Science, Vol. 5, 1976, pp. 245-73. The gel from which the aerogel is produced is either in the form of an aquagel or an alcogel (alcohol as the solvent).
We have discovered that the finely divided porous inorganic oxides can be produced in conventional spray drying apparatus, without the need for high pressures and temperatures by using organic solvents to produce a gel or solution. The pore volumes and surface areas of the spray dried xerogel products are unexpectedly improved as compared to xerogels produced from aquagels, and in some cases approach the textural characteristics of an aerogel.